testsubject
Member
Hi guys!
I have been spending quite a bit of time yesterday and today trying to get a definitive answer to this question regarding protection an enclosure.
I have combed through the NFPA 79, UL-508a, NFPA70, and other documents and have not come up with anything concrete.
Here is an example of the question.
I have a control panel with (4) Powerflex525s: 480VAC, 3P, 0.5hp. Each VFD will have (1) 0.125hp (0.4A) motor attached.
The PF525s max input rating is 1.9A.
The wiring and line breakers installed for the branch are rated for 1.9A.
The question comes up when calculating the feeder FLA and protection.
At a two previous employers, the philosophy was to add up all the input ratings of the drives and call that the FLA and use this times 1.25 for the disconnect fusing. (This would mean 1.9*4=7.6 and I would use 10A fuses)
At my current employment, they add up all the motor FLAs*150% and call that the FLA and fuse appropriately. (This would mean 0.4*4*1.5=2.4 and I would round to 3.)
I do not think this is adequate but need to have documentation to back me up.
We are a UL shop (as were the two previous employers) and UL never mentioned that there is a problem with this assessment at any of these places.
I am aware of Article 430-122 but that only applies to the branch.
I hope this was clear enough. I am trying to find documentation to point to one way or another (or a completely different way I am unaware of….)
Thanks in advance for your input.
I have been spending quite a bit of time yesterday and today trying to get a definitive answer to this question regarding protection an enclosure.
I have combed through the NFPA 79, UL-508a, NFPA70, and other documents and have not come up with anything concrete.
Here is an example of the question.
I have a control panel with (4) Powerflex525s: 480VAC, 3P, 0.5hp. Each VFD will have (1) 0.125hp (0.4A) motor attached.
The PF525s max input rating is 1.9A.
The wiring and line breakers installed for the branch are rated for 1.9A.
The question comes up when calculating the feeder FLA and protection.
At a two previous employers, the philosophy was to add up all the input ratings of the drives and call that the FLA and use this times 1.25 for the disconnect fusing. (This would mean 1.9*4=7.6 and I would use 10A fuses)
At my current employment, they add up all the motor FLAs*150% and call that the FLA and fuse appropriately. (This would mean 0.4*4*1.5=2.4 and I would round to 3.)
I do not think this is adequate but need to have documentation to back me up.
We are a UL shop (as were the two previous employers) and UL never mentioned that there is a problem with this assessment at any of these places.
I am aware of Article 430-122 but that only applies to the branch.
I hope this was clear enough. I am trying to find documentation to point to one way or another (or a completely different way I am unaware of….)
Thanks in advance for your input.